Certain types of mechanical positioning systems have individual elements which are driven by a motor actuator. A typical such motor actuator comprises an electric motor driving a gear train having a power output shaft which positions the mechanical system. (The word "shaft" is used here, but it should be understood that linear motor actuators are also included. Throughout the discussion of the invention to follow the term "member" will be used to include both shafts driven in rotation and arms or racks driven linearly.) Typically the motor actuator is reversible so that the mechanical system ca be theoretically positioned at any orientation or position within the range of allowed motion. An example is the mechanical system which controls the flow of fuel and air to a burner such as is used in a furnace or boiler. It is important that the linkage be adjusted so that the proper stoichiometric ratio is maintained as closely as possible at all firing levels.
In such positioning systems it is frequently required during installation or maintenance to adjust the relationship between the various mechanical outputs of the system at a number of positions of the motor actuator power output shaft. Accordingly, it is convenient to be able to position the power output shaft at any desired orientation.
Heretofore the shaft position has typically been adjusted by use of a potentiometer which is switched into the circuit to replace the control resistance. There are a number of problems with this approach. The potentiometer, being an analog device, does not allow precisely incremented position changes. The accuracy with which the shaft can be position can be positioned depends upon the skill of the operator in adjusting the potentiometer. Frequently, the potentiometer may, after a period of years, become worn or dirty resulting in non-continuous changes in the potentiometer resistance. Lastly, it is also difficult to determine the shaft position as a function of potentiometer setting. Calibration of the potentiometer control shaft to indicate motor actuator shaft position is difficult and unreliable. Lastly, recent changes in the technology to solid state transducers and microprocessor-based controls tend to eliminate the control resistance entirely, and therefore the manual adjustment potentiometer approach is becoming obsolescent.
U.S. Pat. No. 4,143,811 shows a typical installation employing control circuitry for a motor actuator driving a flue damper as well as a valve controlling gas flow to a combustion chamber. Such a system can employ a potentiometer input to the control element to orient the position control system in the manner described as prior art above.